Overbased calcium sulfonate greases have been an established grease category for many years. One known process for making such greases is a two-step process involving the steps of “promotion” and “conversion.” Typically, the first step (“promotion”) is to react a stoichiometric excess amount of calcium oxide (CO) or calcium hydroxide (Ca(OH)2) as the base source with an alkyl benzene sulfonic acid, carbon dioxide (CO2), and with other components to produce an oil-soluble overbased calcium sulfonate with amorphous calcium carbonate dispersed therein. Typically the second step (“conversion”) is to add a converting agent or agents, such as propylene glycol, isopropyl alcohol, water, formic or acetic acid, to the production of the promotion step, along with a suitable base oil (such as mineral oil), to convert the amorphous calcium carbonate to a very finely divided dispersion of crystalline calcium carbonate, also known as a colloidal dispersion, that interacts with the calcium sulfonate to form a grease-like consistency. Such overbased calcium sulfonate greases produced through the two-step process have come to be known as “simple calcium sulfonate greases” and are disclosed, for example, in U.S. Pat. Nos. 3,242,079, 3,372,115, 3,376,222, 3,377,283 and 3,492,231.
It is also possible to combine the promoting and conversion steps into a single step by carefully controlling the reaction. In this one-step process, the simple calcium sulfonate grease is prepared by reaction of an appropriate sulfonic acid with either calcium hydroxide or calcium oxide in the presence of carbon dioxide and a system of reagents that simultaneously act as both promoter (creating the amorphous calcium carbonate overbasing by reaction of carbon dioxide with an excess amount of calcium oxide or calcium hydroxide) and converting agents (converting the amorphous calcium carbonate to very finely divided crystalline calcium carbonate). Thus, the grease-like consistency is formed in a single step wherein the overbased, oil-soluble calcium sulfonate (the product of the first step in the two-step process) is never actually formed and isolated as a separate product. One-step processes are disclosed, for example, in U.S. Pat. Nos. 3,661,622, 3,671,012, 3,746,643, and 3,816,310.
In addition to simple calcium sulfonate greases, calcium sulfonate complex greases have been prepared. These complex greases are typically produced by adding a strong calcium-containing base, such as calcium hydroxide or calcium oxide, to the simple calcium sulfonate grease produced by either the two-step or one-step process, and reacting with stoichiometrically equivalent amounts of complexing acids such as 12-hydroxystearic acid (12-HSA), boric acid, acetic acid, or phosphoric acid. Some advantages of calcium sulfonate complex greases over the simple greases include reduced tackiness, improved pumpability, and improved high temperature utility. Calcium sulfonate complex greases are disclosed, for example, in U.S. Pat. Nos. 4,560,489, 5,126,062, 5,308,514 and 5,338,467.
Calcium oxide or calcium hydroxide can be used as the source of basic calcium for production of calcium sulfonate greases or as a required component for reacting with complexing acids to form calcium sulfonate complex greases. However, the presence of calcium carbonate as a separate ingredient or as an “impurity” in the calcium hydroxide or calcium oxide, other than that presence of the amorphous calcium carbonate dispersed in the calcium sulfonate after carbonation, is undesirable for at least two reasons. The first reason is calcium carbonate is generally a weak base, unsuitable for reacting with complexing acids. The second reason is the presence of unreacted solid calcium compounds (including calcium hydroxide or calcium oxide) interferes with the conversion process, resulting in inferior grease compounds if the unreacted solids are not removed prior to conversion or before conversion is completed.
It is desirable to have calcium sulfonate complex grease compositions with worked penetration of National Lubricating Grease Institute (NLGI) categorization 1-3, suitable dropping points for higher temperature applications and lower amounts of overbased calcium sulfonate to reduce material cost. It is further desirable to have methods of manufacture for such greases that reduce process time and employ open kettle manufacturing processes to eliminate the need for pressure reactors.